Several standards have been developed for data communications over broadband wireless links. One such standard has been set out by the IEEE 802.16 specifications and is commonly known as WiMAX. IEEE 802.16 for example defines wireless communication systems in which a Base Station (BS) communicates with Mobile Stations (MSs). The data is communicated by exchanging packets between the MSs and their respective BS. The direction of transmission of packets from the MS to the BS is referred to as uplink (UL), while the direction of transmitting packets from the BS to the MS is referred to as downlink (DL). The packets have a defined format which follows a layered protocol applied to the system and its components. The protocol layers relevant to packets called physical layer (PHY) and media access layer (MAC).
The MAC layer is used for handling various functions including bandwidth allocation, network access, and maintaining of connections. This includes controlling access of the BS and MSs to the radio medium on the basis of “frames” which are predetermined units of time in the system, and which are divided in the time and frequency domain into a number of “slots”. The PHY layer is applied for the transmission technique utilized, such as OFDM (orthogonal frequency division multiplexing) or OFDMA (orthogonal frequency division multiple access). In OFDM, a single data stream is modulated onto N parallel sub-carriers, each sub-carrier signal having its own frequency range, and the sub-carriers are orthogonal in a mathematical sense so that the sub-carriers' spectra may overlap without interference due to the fact they are mutually independent. This allows the total bandwidth (i.e. all the data that should be sent within a given interval of time) to be divided over a plurality of sub-carriers. OFDMA is a multiple access variant of OFDM. It works by assigning a sub-set of the sub-carriers to an individual subscriber. This allows simultaneous transmission from several users leading to better spectral efficiency.
One of the problems associated with such type of communications is how to ensure the existence of bi-directional (UL and DL) communications without interference. There are two common approaches to overcome the physical limitation by which a radio based device cannot simultaneously transmit and receive on the same resource medium. The first, frequency division duplexing (FDD), according to which the transmission medium is sub-divided into two distinct bands, each operative at a different frequency band, one for DL and the other for UL. The second, time division duplexing (TDD), involves operating the two links at the same frequency band, but sub-dividing the access to the medium in time so that only either the DL or the UL transmissions may utilize the medium at any given point in time.
OFDMA provides a number of “sub-carrier allocation” schemes that define how the physical sub-carriers are grouped into logical sub-channels. One frame can employ several transmission techniques within separate “zones” in the time dimension, i.e. each frame is divided into DL and UL sub-frames. On the downlink transmissions, a single burst may be shared by several users but on the uplink transmissions, each burst generally corresponds to a single user. The DL sub-frame includes a broadcast control field with a DL-MAP and UL-MAP, by which the BS informs the user device of the frame structure. The MAP is a map of bandwidth allocation within the frame and also contains other PHY signaling related messages. It consists of Information Elements (MAP IEs). The MAP IEs inform mobile stations to which burst(s) their connections have been assigned to transmit and receive information. Thus, in a TDD and FDD mode network, bandwidth allocation means the allocation of resources within the frames.
The DL sub-frame has a “2-D” structure, having a defined extent in both frequency and time dimensions. Thus, the MAP provides the MS with information on the part of the frequency band to be used as well as the portion of the sub-frame duration. In most cases, the definition of the UL allocation is simpler than defining the DL allocation, as only a duration parameter is required. Thus the amount of MAP resources allocated for downlink connections is typically many times larger than for the uplink. Notwithstanding the above, in the case of using HARQ allocations in the DL direction, only the duration field within sub-burst allocation IE would be required. The DL sub-frame is divided into two main content parts. One is used for the frame MAPs while the second part is mainly used for DL data allocations. The MAPs part of the DL sub-frame is usually located within a first zone and contains the MAP IEs as well as the MSs' allocations and their related attributes such as MCS (i.e. “modulation coding scheme”), duration, etc. Most of the 802.16e frame MAPs consist of compressed basic MAPs transmitted while using robust MCS to allow coverage for all MSs within the sector. Some frames comprise sub-MAPs located at the end of compressed MAPs and transmitted with higher MCS and holding information elements for allocation of resources to MSs supporting higher DL MAP rates. Using sub-MAPs within a frame enables saving on frame resources otherwise required for the transmission of MAPs, leaving more resources available for DL data transmissions.
A number of uses for sub-MAPs are mentioned in the art:
US 2007086370 discloses a method for allocating transmission periods in a wireless network system, by providing more than one MAP (i.e. MAPs and sub-MAPs) frames to indicate downlink and uplink periods allocated by an access point to each station within a transmission frame period. The sub-MAP allocated period starts immediately after the corresponding MAP frame is transmitted, and within the transmission frame period, the sub-MAP frame is used for error recovery of downlink and uplink data. According to this disclosure, the sub-MAPs are used to retransmit the data transmitted on the downlink, and to transmit acknowledgment of the data received on the uplink.
US 2008205258 describes a method for transmitting and receiving MAP information in a communication system. According to this disclosure, the base station acquires channel quality information of each mobile station, generates sub-MAPs using MAP information separately for each mobile station according to the channel quality information, and transmits the generated sub-MAPs. The mobile station receives MAP information from the base station, detects a sub-MAP allocated thereto from the received MAP information, and restores the detected sub-MAP to MAP information using the same scheme applied in the base station, based on channel quality information of the mobile station.